Numerous known energy conversion devices comprise a solid, cation-permeable casing which is a barrier to mass liquid transfer. In such devices, in order to attach the casing to the remainder of the cell or device, the casing is attached or sealed to a sealing member which, in turn, is attached to other components. Such energy conversion devices generally include anodic and cathodic reaction zones. The anodic reaction zone generally is bounded, at least in part, by a major surface of the casing, and, in preferred devices, contains molten alkali metal which serves as an anode-reactant and is in electrical contact with an external circuit. The cathodic reaction zone of such devices generally contains an electrode which is in electrical contact with both the cation-permeable casing and the external circuit. Also, this zone is bounded, at least in part, by the major surface of said casing opposite the major surface bounding the anodic reaction zone.
Among the numerous well known energy conversion devices falling within the above mentioned general class are: (1) primary batteries employing electrochemically reactive oxidants and reductants in contact with and on opposite sides of the cation-permeable casing; (2) secondary batteries employing molten electrochemically reversibly reactive oxidants and reductants in contact with and on opposite sides of the cation-permeable casing (e.g., the sodium sulfur battery); (3) thermoelectric generators wherein a temperature and pressure differential is maintained between anodic and cathodic reaction zones and/or between anode and cathode; and (4) thermally regenerated fuel cells.
Many electrical conversion devices discussed above and a number of materials suitable as cation-permeable casings are disclosed in the following exemplary U.S. Pat. Nos. 3,404,035; 3,404,036; 3,413,150; 3,446,677; 3,458,356; 3,468,709; 3,468,719; 3,475,220; 3,475,223; 3,475,225; 3,535,163; 3,719,531; 3,811,493; 3,985,576; 4,020,134; 4,048,393; 4,039,889; 4,084,041; and 4,091,190. All of the aforementioned patents are commonly assigned with this application.
In energy conversion devices of the type described above it is critical that the cation-permeable barrier be sealed to the remainder of the device in such a manner as to prevent both ionic and electronic current leakage between the two reaction zones of the device. This insulation insures that the ionic conduction takes place through the cation-permeable casing while the electronic conduction accompanying the chemical reaction follows the external shunt path, thus resulting in useful work. In those instances where one or more reservoirs for cell reactants are employed such that reactants may flow therefrom into a reaction zone created by the cation-permeable casing, it is of course necessary to seal both the casing and the reservoir with which it communicates in an insulative fashion.
In addition to providing necessary insulation, the seal of the cation-permeable casing to other cell components, (i) must support loads to which the casing may be exposed, (ii) should in no way introduce deleterious properties into the electrical conversion device, and (iii) must withstand a variety of environments varying both in temperature and corrosive nature. Thus, while long life and trouble-free operation of a high temperature energy conversion device such as a sodium sulfur battery depends on many factors, including durability of the solid electrolyte, corrosion resistance of the container, stress present in mating various components, etc., one of the most important factors relates to the sealing of the cation-permeable casing to other components of the cell.
Prior art seals for sealing cation-permeable casings to other components of an energy conversion device have been generally of two types. Both employ alpha-alumina ceramic. One of the designs involves use of alpha-alumina in the tubular form (see U.S. Pat. No. 3,985,576) while the other employs an alpha-alumina disc. In both prior art sealing techniques, the cation-permeable casing, such as the beta alumina tube which serves as a solid electrolyte for the sodium sulfur cell, is sealed to either the tube or the disc using a boro silicate glass. In the case of the tube-type seal, the assembly is then sealed to other parts of the battery using a glass seal. In the case of the disc, it is common to employ a mechanical compression seal.
While the prior art seals maintain a reasonably good resistance to attack by cell reactants, such as sodium, sulfur and sodium polysulfide, cell failure frequently occurs either at the seal or it can be traced to the seal area. Seal failure generally occurs during any one of the following stages: (1) cell assembly, (2) cell filling, (3) cell freezing and thawing, and (4) cell operation. Many failures are apparently due to residual stresses incorporated during sealing or stresses created during cell assembly. Misalignment strains occur to some extent and cannot be avoided. The problem becomes particularly critical when designing cells with long cation-permeable casings (e.g., beta-alumina tubes).
It is an object of the present invention to provide a sealing member which not only provides necessary insulation and support for the cation-permeable casing while providing a connecting means for assembling the cell, but also is more flexible so as to take up thermal and misalignment strains and thereby avoid failure of the cell as a result of seal failure.